The attributional reformulation of the learned helplessness model of depression claims that individuals who habitually explain the causes of bad events in internal ("It's me"), stable ("It's going to last forever"), and global ("It's going to undermine everything I do") should suffer more depression following bad events than individuals with a more optimistic explanatory style. This proposal will try to answer five questions which follow from this theory. First, does the change in explanatory style during therapy predict and precede, or merely correlate with relief from unipolar depression? Toward this end, session by session changes in explanatory style will be measured along with changes in depression. Second, does change in explanatory style bear a different relationship to relief from depression in cognitive as opposed to tricyclic anti-depressant therapy? Toward this end, session by session changes in explanatory style and depression will be examined during cognitive and drug therapies. Third, does the amount and quality of cognitive therapy relieve depression by means of changing explanatory style? Toward this end, session by session changes in depression, explanatory style, and therapists' actions will be examined. Fourth, does explanatory style at the end of therapy for unipolar patients predict time to relapse? Toward this end, we will follow two groups of remitted unipolar patients, for 2 to 4 years, attempting to predict whether, and when, they relapse. Fifth, do changes in explanatory style among rapid cycling bipolar depressives precede and predict differentially the onset of manic and depressive episodes? Toward this end, we will follow 30 rapid cycling bipolar patients for 2 years attempting to predict timing and direction of affective episode.